418 THE NERVOUS SYSTEM. 
His remarks, the nerve-cell is therefore the genetic centre from which all the parts 
of a nervous element proceed. 
It must now be clear that each nerve-cell is a unit which is separate and distinet 
from the nerve-cells which le around it. Further, it is obvious that it is wrong to 
consider the nerve-cell as something apart from the nerve-fibre. The nerve-cell 
with its dendrites and axon, however wide-spreading these processes may be, con- 
stitutes an independent system to which the term neuron is applied, and the only 
relation which it has with other neurons or with peripheral tissues is one of contact. 
Ganglionic Nerve-Cells.—The ganglionic neurons found in the gangha of the 
cranial nerves and in the ganglia on the dorsal roots of the spinal nerves have a 
different origin, and present many points of contrast with neurons in the gray matter 
of the brain and cord. As already indicated in the chapter on Embryology (p. 20) 
the ganglia in question are derived from the neural crest. The cells forming these 
ganglionic masses are somewhat oval in form, 
and from either extremity or pole a process 
crows out, and the neurons in this manner 
become bipolar. These processes are dis- 
tinguished as central and peripheral, according 
to the direction which they take. The central 
processes grow inwards, and penetrate the 
wall of the neural tube. In the region of the 
spinal cord they form almost the whole of the 
fibres which enter into the composition of the 
dorsal roots of the spinal nerves. In the 
substance of the cerebrospimal axis they give 
off numerous collaterals, and after a course of 
varying extent they end, after the manner of 
an axon, in terminal arborisations, which enter 
into relationships of contact with certain 
nerve-cells in the cerebrospinal axis. The 
peripheral processes grow outwards along the 
path of the particular nerve with which they 
are associated, and they finally establish peri- 
pheral contact relations. Thus, to take one 
example: the majority of the fibres which go 
ciao emcee , did (i to the skin break up into fine terminal fila- 
a : GRAM OF THE CONNEXION ESTAB- . . “ 
LISHED BY A GANGLIONIC AND a Motor ments, which end freely between the epithelial 
Neuron (Ramon y Cajal). cells of the epidermis. The two processes of 
A. Fibre coming down from a pyramidal cell in the q ganglion cell, therefore, form the afferent 
motor area of the cerebral cortex. 
B. Motor cell in gray matter of spinal cord. fibres of the cerebrospinal nerves, and con- 
C. Muscle-fibres. } } ] hi i ] fi f 
D. Collateral branch from the pyramidal fibre. stitute the pat la ong which the influence o 
E. Cell in the medulla oblongata sending its axon peripheral impressions is conducted towards 
upwards to the cerebral cortex. 
F. Cells in spinal ganglion. _..._. the brain and cord. The body of the cell is 
G. Peripheral process of ganglionic cell ending in skin. : Bays : 2 
I. Collateral branches of central process of gan- as 1t were interposed in the path of such 
peers impulses, 
But the original bipolar character of these 
cells, with very few exceptions (ganglha in connexion with the auditory nerve and 
the bipolar nerve-cells in the olfactory mucous membrane), gradually undergoes a 
change which ultimately leads to their transformation into unipolar cells. This is 
brought about by the tendency which the cell-body has to grow to one side, viz. the 
side towards the surface of the ganglion (v. Lenhossek). This unilateral growth 
leads to a gradual approximation of the attached ends of the processes, and finally 
to a condition in which they appear to arise from the extremity of a short common 
stalk in a T-shaped manner (Fig. 312). It is interesting to note that the original 
bipolar condition of these cells is retained throughout life without change in 
certain fish. 
30th the central and peripheral processes of these ganglionic cells become the — 
axis cylinders of nerve-fibres, which, acquiring a medullary sheath, belong therefore 
to the medullated variety. From this it might very naturally be thought that the — 
